Variable-pitch propellers are widely used in many differing types of aircrafts having power plants ranging from piston engines to gas turbines. Reversing the propellers thrust provides braking power after the aircraft touches down during a landing maneuver, which is especially useful for example, for military transport aircrafts that occasionally need to land in provisional short runways.
During forward flight, the propeller blades are positioned to have positive pitch to provide forward thrust for the airplane. To speed down the aircraft during landing, or to facilitate pivot turns during taxing, the propellers thrust is reversed, such as the propeller blades are rotated about their axes until they have negative pitch angle to provide reverse thrust.
Normal practice is to rely on the pilot skills for the transition to the high drag reverse propeller pitch and the synchronization in opposite power plants. Typically, for the thrust reversal operation the pilot first changes the throttle lever position in order to reduce fuel flow to the engine to reduce engine speed. Then, the pilot reverses the pitch of the propellers and after that, the pilot restores the throttle lever to a position of higher fuel flow in order to resume a higher engine speed. During this operation, the pilot monitors the speed of the engine in order to assure that neither the speed of the engine nor that of the propeller becomes excessive.
However, this practice is subject to human errors because it relies entirely on the pilots skills, and does not prevent other asymmetries caused by delays due to engine/propeller response to control commands or malfunctions.
As a result, yaw moments can then be generated creating discomfort and, most importantly, compromising the aircraft safety and increasing the pilots workload to keep the aircraft within the runway.
Discomfort and yaw is magnified in aircrafts having four power plants due to the larger distance between the outboard engines. In many cases the solution is to enlarge the aircraft vertical tail but this solution has a significant impact in weight, fuel consumption, and aircraft performance.
Even being of less magnitude, yaw is also produced by the asymmetry created between the two inboard engines in a four engine aircraft and in a two engine aircraft. Although this is not normally the sizing parameter for the aircraft tail, its prevention could also be considered beneficial, provided other aircraft performance, as the landing or RTO (rejected take off) distance, is not significantly compromised.
When the aircraft is on ground, engines are at idle power and blades have a positive angle, and little drag is generated by the propellers. However, when propeller blades transition to negative angles (commanded by the Aircraft Power Levers), even though the engine is at idle, drag generated by propeller increases considerably. Therefore, a significant drag asymmetry is produced if two opposite engines do not transition to negative pitches simultaneously.
FIG. 1A illustrates that type of situation (A) where the outboard engine number (1) goes to negative pitch zone, while the opposite engine (engine number (4)) remains at low power positive pitch zone. The effect (B) of this situation (A) is that a thrust asymmetry is originated, which in turn produces a significant yaw moment at high speed of the aircraft.
FIG. 1B illustrates another type of situation (A) where one of the opposite engines, engine number (1) fails before or during the thrust reversal operation. The effect (B) of this situation (A) is again that a thrust asymmetry is created, which in turn produces a significant yaw moment at high speed of the aircraft. The same situation of FIG. 1B occurs if one of the engines is accidentally ordered to reverse thrust.
Therefore, a need has been detected in this technical field for a propeller control method that assures the simultaneous or symmetrical transition to reverse thrust in twin propellers.